This invention relates to a safety module, and more particularly to a safety module which provides fail-safe operation of an exercise treadmill speed control in order to prevent high speed operation of the treadmill drive motor when the speed control thereof fails.
With the advent of new transistor technology, including high power MOSFET's and IGBT's, a new class of low-cost, highly efficient variable speed DC motor controls has been developed. Such variable speed motor controls are commonly used to control the operating speed of the moving tread of an exercise treadmill. The motor controls are pulse width modulated (PWM) DC motor speed controls, which are replacing standard phase-controlled SCR-type speed controls in some treadmill applications.
When a PWM (pulse width modulated) motor speed control is used in an application where an operator is in direct interaction with the machine, such as in an exercise treadmill, if the power transistor in the PWM motor speed control unit fails in a short circuited mode, then instantaneous full output will be applied to the DC motor, thereby abruptly increasing the speed of the treadmill to substantially full speed and possibly causing physical harm to the user and/or damage to the equipment. In the prior art full-wave SCR motor speed controls, if the SCR of the control were to fail, the control output would normally go to zero, and the treadmill would stop, thereby reducing the risk associated with failure of the speed control.
When using a PWM motor speed control for a moving tread of a treadmill, it is advised that an additional safety circuit, such as a special fast-acting fuse, a speed monitoring circuit, or other safety device be used in conjunction with the PWM speed control in order to prevent physical harm to the user and/or damage to the equipment if the power transistor thereof fails in a short circuited mode. However, such prior controls have been found to be insufficient.